Introduce an automatic code style formatter

[Ailrun]

IMO, Current code style is sufficiently inconsistent to introduce a tool for managing it.

~I found that ocamlformat is not so bad in terms of the number of options, reliability, etc.~

How about introducing it and formatting codes with it?

[Ailrun]

I tried ocamlformat and ocp-indent, and no one work really well...

1. ocamlformat There are too few options to control code style reasonably. It introduces a lot of conflict between formatted codes and tuareg mode indentation.
2. ocp-indent It does not remove redundant spaces, inconsistent linefeed, etc. Furthermore, it also creates issues related to indentation.

[tsani]

Yes, automatic style checking / fixing seems like a tough issue to tackle, tbh. I'm not sure how to handle it.

[MartyO256]

An ocamlformat configuration was added in #266. However, most files in the core library and Harpoon are not formatted, and are instead in an .ocamlformat-ignore file. Automatic formatting may be enabled later for new modules.